1. Field of the Invention
This invention relates in general to a servo circuit for use with an apparatus for reproducing information recorded on a disk and in particular to a servo circuit for use with apparatus for reproducing information on a disk such as PCM (pulse code modulation) information on an audio disk wherein a digital signal is recorded.
2. Description of the Prior Art
Signal detection systems for PCM (pulse code modulation) audio disk such as optical and electrostatic capacitive types are known in the prior art.
Two methods have been proposed for recording audio PCM signals on a disk. In one system the information is recorded on the disk which is rotated at a constant angular velocity and the other system is to record information on the disk which is rotated at a constant linear velocity. So as to increase the recording density it is preferable to record the audio PCM signals on the disk at a constant linear velocity. For this case, the disk on which the audio PCM signal is recorded at a constant linear velocity must be played back such that the disk is rotated at a constant linear velocity.
It is known in the prior art to control a disk so as to rotate it at a constant linear velocity during the playback mode in which the position of a pickup device is detected with a potentiometer and the detected output is delivered to a divider so as to provide control information since the necessary number becomes an inverse number for that position. In this method, an arrangement consisting of the position detector and the divider for controlling rotation of the disk are very expensive and complicated.
In order to obviate these defects other apparatus has been proposed wherein the detector for detecting the position of the pickup is eliminated and a playback signal from the disk is utilized to control the disk so as to rotate the disk at a constant linear velocity.
In examples of such conventional apparatus during recording and audio PCM signal it is recorded with a base band with a carrier modulation system such as a AM (amplitude modulation) system and a FM frequency modulation system and the modulation method using a run length limited code is usually employed. Such run length limited code modulation method is used to raise the recording efficiency by extending or prolonging the minimum transition interval T.sub.min between the transition of two data bits representative of "0" and "1" which is the minimum interval of a period during which the signal is inverted and the system results in a self-clock on the playback apparatus by shortening the maximum transition interval T.sub.max which is a maximum interval during which the period is inverted.
In such prior art apparatus, the deviation or displacement of the maximum transition interval T.sub.max are the minimum transitional interval T.sub.min from a reference value when a linear velocity is utilized as a reference which is detected and used as information to drive the velocity servo.
By utilizing the fact that the modulation output where the maximum transition interval T.sub.max is continuous and is not generated by normal modulation, a bit pattern in which such maximum transitional interval T.sub.max occurs twice is employed as a frame synchronizing signal. Thus, utilizing this frame synchronizing signal which always appears during one frame period without fail allows the rotation of the disk or motor to be controlled such that the maximum transition interval T.sub.max equals the reference value.
As an example, the maximum transitional interval T.sub.max can be selected as 5.5 T where T represents a bit cell period of an input data.
FIG. 1 schematically illustrates an example of the conventional reproducing apparatus of the prior art which is applied to an apparatus for reproducing a disk of an optical signal detection system.
In FIG. 1, a photo detector 1 reproduces a playback PCM signal S.sub.p which has a wave form which is distorted and is then an almost sine wave shape. The signal S.sub.p is supplied to an amplifier 2 and then to a wave form converting circuit or converter 3 which obtains a signal S.sub.0 illustrated in FIG. 2A which indicates the "1" and "0" of the recorded signal and this output signal is supplied to output terminal 4. The signal S.sub.0 is also supplied to an edge extracting circuit 5 which comprises a differentiating circuit and which produces a signal PI illustrated in FIG. 2B which indicates the rising up and falling down edges of the signal S.sub.0.
The signal PI is supplied to the base of a transistor 6T which serves to reset an integrating circuit or integrator 6. The integrator 6 has a current I which flows from the constant current supply source 6A to a capacitor 6C which charges and discharges and this current charges the capacitor 6C and the voltage across the capacitor 6C will be raised to a high value with a constant charging rate. When the pulse PI is applied to the transistor 6T, the transistor 6T will be turned on by pulse PI which will cause the voltage on the capacitor 6C to be instantly discharged through the transistor. Thus, the integrator 6 produces a sawtooth-wave voltage SA illustrated in FIG. 2C which has peaks equal to the level corresponding to the length of the transition interval of the signal S.sub.0.
The sawtooth wave voltage SA is supplied through a buffer amplifier 7 to a peak value hold circuit 8. The output voltage of the buffer amplifier 7 is supplied through a diode 8D to a capacitor 8C to charge it and the capacitor 8C is discharged at a time constant determined by a resistor 8R and the capacitor 8C. The discharge time constant is selected to be several to 10 times the repeating period of the data frame of the playback signal so that the peak value of the sawtooth wave voltage SA will be held by the capacitor 8C.
Since the frame synchronizing signal appears once for every one period of the data frame, the output signal PH illustrated in FIG. 2D obtained from the peak value hold circuit 8 maintains a level corresponding to the length of the maximum transition interval T.sub.max within the playback signal. Thus, if the level of the signal provided by the peak value hold circuit 8 when the maximum transition interval T.sub.max is exactly the correct transition interval 5.5 T is taken as a velocity reference voltage E.sub.S and the difference in level between the velocity reference voltage E.sub.s and the output signal PH of the peak value hold circuit is detected it will indicate the amount of the deviation of the rotation of velocity of the disk varies from linear velocity when recording.
For this reason, the output signal PH of the peak value hold circuit 8 is supplied to a comparator 9 in which the difference from the velocity reference voltage E.sub.S is detected. A spindle motor for rotating the disk is controlled by the output signal from the comparator 9 and, thus, the disk will be adapted so that it rotates at a constant linear velocity using the playback signal.
However, in this apparatus, if a scratch is formed on a disk or dust adheres to the disk, there will be a dropout in the playback signal and also when a track is jumped during the playback mode, the signals illustrated in FIGS. 2A through 2D will not occur and, thus, the pulse PI will not exist at the output of the edge extracting circuit 5 for this period. However, the capacitor 6C in the integrator 6 will continue to be charged and the output voltage PH of the peak value hold circuit 8 will gradually increase which will cause the spindle motor 10 to drive at an inaccurate speed.